Aromatic dicarboxylic acids, such as terephthalic acid and isophthalic acid, are used to produce a variety of polyester products, important examples of which are poly(ethylene terephthalate) and its copolymers. These aromatic dicarboxylic acids are synthesized by the catalytic oxidation of the corresponding dialkyl aromatic compounds, which are obtained from fossil fuels. There is a growing interest in using renewable resources as feed stocks in the chemical industry, mainly due to the progressive reduction of fossil fuel reserves and their related environmental impact.
FDCA is a versatile intermediate considered as a promising bio-based alternative to terephthalic acid and isophthalic acid. Like aromatic diacids, FDCA can be condensed with diols such as ethylene glycol to make polyester resins similar to polyethylene terephthalate (PET). FDCA has been prepared by oxidizing 5-(hydroxymethyl)furfural (5-HMF) with air using homogenous catalysts, but only a maximum yield of 44.8% was reported using a Co/Mn/Br catalyst system and a maximum yield of 60.9% was reported using a Co/Mn/Br/Zr catalyst system.
Recently, we reported a process for producing FDCA in high yields by liquid-phase oxidation of 5-HMF using a Co/Mn/Br catalyst system. The process minimizes solvent and starting material loss through carbon burn (U.S. patent application Ser. No. 13/228,803, filed on Sep. 9, 2011; the entire content of which is hereby incorporated by reference).
Heterogeneous-catalyzed oxidation of 5-HMF using ZrO2 mixed with platinum (II) acetylacetonate in water has also been reported (U.S. Pat. No. 7,700,788 B2), but due to FDCA's very low solubility in water, this process has to be conducted under very dilute conditions to avoid the FDCA from precipitating on the catalyst's surface, which makes the process uneconomical. Another heterogeneous-catalyzed oxidation of 5-HMF has been reported using molecular O2 and a Pt/C catalyst (U.S. Pat. No. 4,977,283). High FDCA yield was achieved, but with the additional expense of feeding purified O2 and continually adjusting pH via sodium hydroxide addition. The reaction product was the disodium salt of FDCA, which leads to a wasteful salt by-product in the conversion to FDCA.
In view of the above, there is a need in the art for a high-yield process for producing a dry, purified FDCA product (e.g., ≧90% FDCA yield). There is also a need in the art for a process for making FDCA that includes solvent and energy recovery from the oxidation zone off-gas stream.
The present invention aims to address these needs as well as others, which will become apparent from the following description and the appended claims.